The present invention relates to a hardness indicator composition for measuring the hardness of industrial water, daily life water and the like as well as a method of water hardness analysis using the hardness indicator.
As is well known, on feed water lines leading to cooling and hotting systems such as boilers, water heaters and cooling towers, devices for removing hardness contained in the feed water are used from the need for preventing scale deposition within the cooling and hotting systems. Among others, automatic regenerative water softeners (hereinafter, referred to as water softener) of the type that hardnesses are removed by using ion exchange resins have been widely used.
Conventionally, as the hardness measurement of the raw of the water softener (generally, high in hardness), there has been available a method of determining the hardness by titration with EDTA (ethylenediamine tetraacetate) by using a metal indicator (Calmagite, EBT (Eriochrome Black T), or the like) which changes in color through reaction with hardness (Ca, Mg, etc.) in an alkaline condition. This method, however, involves titration with a two-liquid system, thus taking much time and labor for measurement. For this reason, there has been a desire for implementation the hardness measurement with a one-liquid system without involving titration with EDTA.
Conventional indicators for use of detection of hardness leakage are effective for detecting hardnesses in the treated water of the water softener. However, those indicators, because of their too narrow range of measurement, cannot be used for the hardness measurement of raw water of the water softener. For this reason, there has been a desire for indicators capable of measuring even high-hardness feed water.
In view of these and other problems, an object of the present invention is to provide a hardness indicator composition, as well as a method of water hardness analysis, which makes it possible to measure the hardness without requiring titration with a two-liquid system and yet to measure the hardness of even high-hardness feed water.
The present invention having been achieved with a view to solving the above problems, according to a first aspect of the invention, there is provided a hardness indicator composition which is formulated of a metal indicator with a desensitizer added thereto.
In one aspect of the invention, the metal indicator contains a dye represented by any one of General formulas (I)-(III): 
wherein R is any one of H, OH, NO2, CH3, SO3H and COOH, 
where R is any one of H, OH, NO2, CH3, SO3H and COOH, and 
where R is any one of H, OH, NO2, CH3, SO3H and COOH.
In an embodiment of the invention, the desensitizer is an organic polybasic acid, or a salt thereof, having two or more carboxyl groups.
And, according to the present invention, there is provided a method of water hardness analysis using an indicator as defined in any one of the aspects of the invention, the method comprising the steps of adding the indicator to a sample solution, and then determining a change in color of the sample solution according to an optical transmittance in a vicinity of a specific wavelength.
Further, according to the present invention, there is provided a method of water hardness analysis using an indicator as defined in any one of the aspects of the invention, the method comprising the steps of adding the indicator to a sample solution, and then determining a change in color of the sample solution according to an optical transmittance in vicinities of two or more specific wavelengths.